The present invention relates to clock circuits and more particularly, to phase-locked loop circuits with varying frequencies.
All electronic equipment inherently generates electromagnetic interference (EMI) of one sort or another. This EMI can affect the operation of other equipment. A common example of this would be placing a toaster next to a TV set. When the toaster is running, strange lines, the EMI, appear in the TV picture. In the United States all electronic equipment must meet rules for electromagnetic emissions laid down by the Federal Communications Commission (FCC). These rules are designed to ensure that EMI does not affect other electronic equipment. The FCC rules specify how much energy a piece of electronic equipment may radiate at any particular frequency.
Any electronic equipment that incorporates digital circuits usually requires a clock signal of some frequency. In sophisticated equipment, such as computers, this clock signal can be of quite high frequency, such as 60 MHz. In addition, this clock often is connected to several components so that the wire or trace on a circuit board can be quite long. A long wire acts as an antenna with the result that the electronic equipment radiate a lot of energy at the clock frequency and its harmonics. This can be troublesome when trying to meet FCC rules.
FIG. 1 shows a spectral plot of energy versus frequency for an ideal clock. All the energy is concentrated at the clock frequency (and its harmonics), The plot also shows the FCC level. This clock exceeds that level. A well known technique to reduce the peak energy is to spread the spectrum of the clock. This technique is used, for example, in spread spectrum radio equipment. By spreading the clock energy over a broader frequency band, then no peak will exceed the FCC level. This is shown in FIG. 2.
Phase-locked loop (PLL) circuits have been used to provide precise clock signals in a variety of applications in the electronics field. The present invention is an improvement over a conventional PLL. According to the present invention, the frequencies of the output signal from the PLL can be precisely controlled. Furthermore, the rate at which the frequency changes can also be controlled.